1. Field of the Invention
The present invention generally relates to wavelength measuring devices, light receiving units, and wavelength measuring methods, and more particularly, to a wavelength measuring device, a light receiving unit, and a wavelength measuring method that can be suitably used in the following fields: the optical communication field in which semiconductor laser diodes and semiconductor LEDs (Light Emitting Diodes) are used; the fields of processing industries using gas lasers, YAG lasers, and medical lasers; the field of optical pickup technology in which writing and reading are performed on general recording media such as DVDs (Digital Versatile Disks) and CDs (Compact Disks); and the fields of general consumer product industries involving infrared rays or the like.
2. Description of the Related Art
In the field of optical communications using semiconductor laser diodes and semiconductor LEDs (Light Emitting Diodes), involving WDM (Wavelength Division Multiplexing) in particular, the communication wavelength region is becoming more overcrowded, as the allowed space is becoming smaller. In this trend, there is a need to develop a high-precision wavelength control technique. Also, in the processing industries utilizing gas lasers, YAG lasers, and medical lasers, high-precision wavelength control is required, as the developments in nanotechnology such as micromachining are becoming more and more active. Further, in the field of optical pickup technology for performing reading and writing on general recording media such as DVDs and CDs, the wavelengths in the visible region are rapidly becoming shorter, as higher-density recording media are being more widely used. In the future, higher-precision wavelength control will be strongly required for the use of multi-wavelength pickup LDs or mixed recording density devices that can perform reading and writing on next-generation recording media. Also, the infrared control technique used in remote controllers has been applied to various fields, and there is an increasing demand for a wavelength control technique that can be used in multi-channel or multimedia remote control.
So as to realize a high-precision wavelength control operation in the above described fields, it is necessary to accurately determine the oscillation wavelength from each light source. As a conventional oscillation wavelength determining technique, Japanese Laid-Open Patent Application Publication No. 2002-340688 (hereinafter referred to as Document 1) discloses the following technique. As shown in FIG. 1, the structure disclosed in Document 1 has photoelectric converter layers 101 and 102 provided in two locations on the light path. The photoelectric converter layers 101 and 102 have different sensitivity characteristics from each other. Based on the ratio between the photoelectric conversion currents outputted from the photoelectric converter layers 101 and 102 (the ratio corresponding to the “sensitivity ratio” in the following), the wavelength is determined.
In the above structure, light receiving elements PD_A and PD_B having different peaks in wavelength sensitivity (the wavelength sensitivities at wavelengths λ_A and λ_B: hereinafter referred to as the “peak wavelengths”) are combined, so that a preferable sensitivity ratio can be obtained in the region between the wavelengths λ_A and λ_B (a wavelength determinable range F), as shown in FIG. 2. Thus, the wavelength can be accurately determined. FIG. 2 is a graph showing the wavelength determinable range F in the case where the light receiving elements PD_A and PD_B with ideally different sensitivity characteristics are combined. The light receiving elements PD_A and PD_B are equivalent to the photoelectric converter layers 101 and 102, and will be hereinafter also referred to as the light receiving elements PD.
By the technique disclosed in Document 1, however, two or more light receiving elements PD are combined, and accordingly, the wavelength determinable range is restricted by the physical characteristics of each light receiving element PD. For example, if the peak wavelengths in the sensitivity characteristics are almost the same, the wavelength determinable range F becomes very narrow, as shown in FIG. 3. If the peak wavelengths in the sensitivity characteristics are wide apart from each other, the wavelength sensitivity of one of the light receiving elements PD cannot be sufficiently obtained at the peak wavelength of the wavelength sensitivity of the other one of the light receiving elements PD, as shown in FIG. 4. In the case shown in FIG. 4, the ratio of the two photoelectric conversion currents cannot be accurately calculated. In another case where the combination of the light receiving elements PD_A and PD_B is not preferable, the wavelength determinable range F does not cover a desired wavelength determinable range F′ at all, as shown in FIG. 5.
So as to obtain the desired wavelength determinable range F′, light receiving elements PD_A and PD_B having the peak wavelengths that are at such a distance from each other as to sandwich the desired wavelength determinable range F′ should be combined. However, the light receiving elements PD to be employed vary with the desired wavelength determinable range F. Therefore, it is very difficult to select suitable light receiving elements PD for every wavelength determinable range F′.
Document 1 also discloses a structure in which the wavelength determinable range F can be varied by controlling the temperatures of the two light receiving elements PD (the photoelectric converter layers 101 and 102). In that conventional structure, however, both of the light receiving elements PD are controlled at the same temperature. When the temperature rises, as shown in FIG. 6, the wavelength determinable range for simultaneous measurement merely moves from F1 to F2, and cannot be widened. Furthermore, if the light receiving element that decides the lower limit (the light receiving element PD_A in the example shown in FIG. 6) exhibits a greater temperature dependency in the wavelength sensitivity, i.e., a greater shifting amount of the peak wavelength with respect to the temperature change, the bandwidth of the wavelength determinable range F2 after the temperature rise is smaller than the bandwidth of the wavelength determinable range F1 prior to the temperature rise. If the light receiving element that decides the upper limit (PD_B) exhibits a greater shifting amount of the peak wavelength with respect to a change in temperature, on the other hand, the bandwidth of the temperature determinable range after the temperature drop is smaller than the bandwidth of the wavelength determinable range prior to the temperature drop.